Recently, a liquid crystal display device has been widely used as a display device of an information device such as a notebook-type personal computer, a word processor and the like, or as a display device of a video device such as a portable television, a video movie, a car navigation system and the like, by taking advantage of a characteristic in which the liquid crystal display device is light and thin, and consumes small electricity. Such liquid crystal display device typically has a structure in which a display element is illuminated from behind by a built-in lighting unit for obtaining a bright display screen.
As a lighting unit, there is an edge light type lighting unit in which a linear light source such as a fluorescent discharge tube is disposed on an end face of a light guiding plate disposed on a rear surface of the display element (an opposite surface of a display surface of the element). The edge light type is characterized in that a thin lighting unit and a highly uniform luminance of a light emitting surface thereof can be obtained. Therefore, in order to give priority to thinness of the lighting unit, the edge light type is commonly adopted in the lighting unit used as a back light of the liquid crystal display device composing the notebook-type personal computer and the like. And, in the liquid crystal display device composing the portable television, the car navigation system and the like, the edge light type comprising two or more linear fluorescent discharge tubes, or the edge light type using a fluorescent discharge tube disposed extensively continuously along an outer periphery of side portions of the light guiding plate, for example, an L-shaped or a U-shaped fluorescent discharge tube, are commonly adopted for the thinness and the luminance to be compatible with each other.
The fluorescent discharge tube used as the light source is driven by a high frequency alternating current to emit light. So, lead wires are respectively connected to both end portions of the fluorescent discharge tube disposed on the side portions of the light guiding plate, for externally supplying a predetermined drive voltage. And, the two lead wires connected to the end portions are provided with connectors at tip end portions thereof and are connected to an inverter externally provided. Since the liquid crystal display device is required to provide improved portability and to save a space, it is important to compactly dispose the light guiding plate, the fluorescent discharge tube, the lead wires, each of which composes the lighting unit, within the lighting unit and liquid crystal display device to allow the device to be light and thin.
FIG. 6 is a plan view schematically showing the conventional edge light type lighting unit, seen from a light emanating side. FIG. 7 is a cross-sectional view schematically showing a structure of the liquid crystal display device comprising the lighting unit shown in FIG. 6. The lighting unit part thereof corresponds to a cross-sectional view taken along line VII–VII′ in FIG. 6. In FIG. 6, to describe the structure, only a part of components of the lighting unit shown in FIG. 7 are shown.
As shown in FIGS. 6 and 7, a lighting unit UT comprises as main components, a light guiding plate 1, a fluorescent discharge tube 2, a reflecting sheet 3, a lower casing 9, and an upper casing 10. A liquid crystal panel 11 and a front cover 12 are disposed on the lighting unit UT to compose a liquid crystal display device LD.
In the lighting unit UT, the L-shaped fluorescent discharge tube 2 is disposed along two adjacent end faces D1 of a flat transparent light guiding plate 1, as the light source. Lead wires 7 are respectively attached to end portions of the fluorescent discharge tube 2 by soldering or the like. Each of the lead wires 7 is disposed along each of two end faces D2 of the light guiding plate 1, on which the fluorescent discharge tube 2 is not disposed, and is connected to a power supply unit 20 such as an inverter disposed outside of the lighting unit UT, through a through-hole H provided on a side surface of the lower casing 9, as described below. When the lighting unit UT emits light, a high voltage is applied through the lead wires 7 for lighting the fluorescent discharge tube 2. For this reason, cylindrical holders 8 made of an insulating material such as rubber are attached to connecting portions between the lead wires 7 and the fluorescent discharge tube 2, for safely applying the high voltage and for protecting electrodes of the fluorescent discharge tube 2.
The two end faces D2 of the light guiding plate 1, on which the fluorescent discharge tube 2 is not disposed, a rear surface (an opposite surface of the light emanating surface) of the light guiding plate 1, and a vicinity of the two end faces D1 of the light guiding plate 1, on which the fluorescent discharge tube 2 is disposed and the fluorescent discharge tube 2 are continuously covered with the reflecting sheet 3. As used herein, reflector portions RF refer to portions including the vicinity of the two end faces D1 of the light guiding plate 1, on which the fluorescent discharge tubes 2 are disposed, and the fluorescent discharge tubes 2. As the reflecting sheet 3, for example, a white resinous film having a high reflectivity is used. The reflecting sheet 3 may be provided with a printed pattern for promoting diffusion of light as it is distant from the fluorescent discharge tube 2. FIG. 8 is an expansion plan of the reflecting sheet 3. As shown in FIG. 8, the reflecting sheet 3 is obtained by cutting in a predetermined shape configured to cover the above-described regions of the light guiding plate 1 and the reflector portion RF, and is provided with fold lines S in predetermined portions so that the sheet 3 is folded so as to correspond to the regions to be covered. Although herein, one reflecting sheet 3 configured to continuously cover the above-described regions is used, the reflecting sheet 3 may be separated into a reflecting sheet 3 covering the reflector portion RF and a reflecting sheet 3 covering the light guiding plate 1, which may be bonded to each other by a double face adhesive tape or the like. However, when the integral-type reflecting sheet 3 is used as shown in FIG. 8, a thin lighting unit, cost reduction thereof, and reduction of the number of assembling processes are effectively achieved.
As shown in FIG. 7, the light guiding plate 1 and the fluorescent discharge tube 2 covered with the reflecting sheet 3, and the lead wires 7 disposed along the end faces D2 of the light guiding plate 1 with the reflecting sheet 3 interposed between the lead wires 7 and the end faces D2 are stored within the lower casing 9 of a rectangular parallelepiped shape. Within the lower casing 9, the lead wires 7 are stored within a space E2 formed by an inner wall of the casing and the end faces D2 of the light guiding plate 1 covered with the reflecting sheet 3. The through-hole H is provided on a predetermined portion of the lower casing 9 to allow the space E2 and outside of the unit to communicate with each other, and the lead wires 7 are pulled out of the lower casing 9 through the through-hole H. The fluorescent discharge tube 2 is stored within a space E1 formed by the inner wall of the casing which substantially contacts the reflecting sheet 3, and the end faces D1 of the light guiding plate 1.
In order to make the lighting unit UT smaller, width M1 of the space E1 in which the fluorescent discharge tube 2 is stored and width M2 of the space E2 in which the lead wires 7 are held, are set to be as small as possible, i. e., a minimum width necessary for the fluorescent discharge tube 2 and the lead wires 7 to be disposed. Herein since a diameter of the lead wire 7 is smaller than that of the fluorescent discharge tube 2, the width 2 of the space E2 is smaller than the width 1 of the space E1. The width M1 of the space E1 corresponds to a distance between the inner wall of the casing 9 and the end face D1 of the light guiding plate 1, and the width M2 of the space E2 corresponds to a distance between the inner wall of the casing 9 and the end face D2 of the light guiding plate 1.
Light correction sheets 4 and 5 are further disposed on the light emanating surface side of the light guiding plate 1 stored within the lower casing 9. The light correction sheets 4 and 5 are not bonded to the light guiding plate 1, and there is a gap between them. As the light correction sheet, a plurality of light diffusion sheets for diffusing light, prism sheets for collecting light or the like are disposed as necessary. Thereby, it becomes possible to obtain a uniform and highly luminous light by diffusing and collecting the light emitted from the light guiding plate 1.
The upper casing 10 as a lid member of the lower casing 9 is fitted to the lower casing 9 so as to store the light guiding plate 1 stored within the lower casing 9 and the light correction sheets 4 and 5 disposed on the light guiding plate 1. In this way, the lighting unit UT is structured. A liquid crystal panel 11 is disposed on the light emanating surface side of the light guiding plate 1 of the lighting unit UT, and the front cover 12 is further attached to compose a liquid crystal display device LD. The liquid crystal panel 11 and the light correction sheets 4 and 5 are not bonded to each other, and there is a gap between them.
When the lighting unit UT and the liquid crystal display device LD structured as described above are operating, the high frequency alternating current generated in the power supply unit (not shown) such as the inverter is given to the fluorescent discharge tube 2 through the lead wires 7. Thereby, the high voltage is applied to the fluorescent discharge tube 2, which is thereby lighted. Light emitted from the fluorescent discharge tube 2 is introduced from the end faces D1 of the light guiding plate 1 into an inside thereof, passes through the inside thereof, and emanates from the emanating surface thereof. Since the light leaking out of the light guiding plate 1 and the fluorescent discharge tube 2 is reflected by the reflecting sheet 3 and is guided into the light guiding plate 1 again, it becomes possible to increase an amount of light emanating from the light emanating surface of the light guiding plate 1. The light emanating from the light guiding plate 1 is diffused and collected by the light correction sheets 4 and 5 and then enters the liquid crystal panel 11 disposed above. In the liquid crystal panel 11, display is performed by using this light. Hereinafter, a region of the liquid crystal panel 11 on which the display is performed and a region located below the above-described region and concerning the display by supplying the above-described region with light are referred to as a display region of the liquid crystal display device LD.
In the conventional lighting unit UT and the liquid crystal display device LD as described above, the light guiding plate 1 and the light correction sheets 4 and 5 are not bonded to each other, and the light correction sheets 4 and 5 and the liquid crystal panel 11 are not bonded to each other, as described above. Therefore, there are gaps between the light guiding plate 1 and the light correction sheets 4 and 5, and between the light correction sheets 4 and 5 and the liquid crystal panel 11. For this reason, dust might enter the gaps. When the dust enters the display region of the liquid crystal display device LD, the emitted light is blocked by the dust, which causes non-uniform luminance to occur. And, since components in the display region such as the light correction sheets 4 and 5 are damaged by a friction between the components and the dust, light emission capability is deteriorated. Once the dust enters a gap between the liquid crystal panel 11 and the lighting unit UT, it is very difficult to clear the dust away without disassembling the panel 11 and the unit UT.
For example, in the above-described lighting unit UT, the through-hole H for pulling out the lead wires 7 is provided on the lower casing 9 to allow an outside of the lighting unit UT and an inside thereof to communicate with each other. Therefore, the dust generated outside of the liquid crystal display device when assembling the same enters the lower casing 9 through the through-hole H provided on the lower casing 9, as indicated by arrow G in FIG. 7, and further enters the display region. Although not shown in FIG. 7, a plurality of through-holes are formed on the lower casing 9 for assembling the same and the upper casing 10, and the dust also enters the inside through these through-holes. Furthermore, the dust existing within the unit also enters the display region in addition to the dust entering from outside of the unit.
Herein, as described above, since the width M2 of the space E2 in which the lead wires 7 are disposed is smaller than the width M1 of the space E1 in which the fluorescent discharge tube 2 is disposed, distance between the entered dust and the display region is shorter in the space E2 than in the space E1. Therefore in the space E2, it is highly possible that the dust enters an effective display region.
One method for solving the above-described problem, another component for blocking entry of the dust may be disposed on a path through which the dust enters (for example, arrow G in FIG. 7). However, the lighting unit UT and the liquid crystal display device LD are required to be light and small as described above, dimension between the light guiding plate 1 and an outer shape of the liquid crystal display device LD and weight of the device are restricted. Consequently, it is difficult to dispose such a component in the vicinity of the light guiding plate 1.